Ultrasonic characterization of cardiac and vascular tissues based on quantitative analysis of backscatter permits detection of pathologic changes in heart muscle and vascular structures in experimental animals and human subjects. Potential applications include quantitative assessment of the efficacy of thrombolysis or angioplasty in salvaging jeopardized myocardium and pharmacologic treatment of hyperlipidemia designed to interrupt the progression of atherosclerotic arterial disease. The hypothesis underlying the proposed research is that pathologic changes in cardiac and vascular tissues produce alterations of physical properties that can be quantified by measuring ultrasonic backscatter and attenuation over a range of frequencies. The basic structural determinants of the interactions between ultrasound and cardiovascular tissue have not yet been defined sufficiently to permit unambiguous interpretation of ultrasonic data acquired from tissue manifesting pathologic derangements in terms of the specific alterations of tissue components responsible for scattering and attenuation. Two ultrasonic parameters, integrated backscatter and frequency-dependence of scattering, define complementary intrinsic scatterer properties that should facilitate characterization of cardiovascular pathology. Quantification of frequency averaged, or in- tegrated, backscatter has shown considerable promise for detection of cardiomyopathy, old myocardial infarction, acute ischemic injury, and atherosclerotic plaque. However, elucidation of the frequency content of scattered ultrasound has received less attention despite its substantial promise. We propose to identify the structures in cardiac tissue that determine scattering behavior under physiologic conditions and the mecha- nisms by which changes in and among these elements reflecting reversible or irreversible ischemic injury alter ultrasonic scattering from myocardium. We propose also to characterize mechanisms of scattering in normal vascular tissue and define ultrasonic features diagnostic for specific stages in the evolution of atherosclerotic plaque. Methods for collection of real-time backscatter data for these purposes in the clinical environment will be implemented with absolute calibration procedures necessary for interpatient comparisons. Elucidation of scattering mechanisms in vitro and in vivo with quantitative ultrasonic indexes should potentiate the utility of ultrasonic tissue characterization for diagnostic and prognostic applications in patients.